tree-optimization/93491 - avoid PRE of trapping calls across exits

This makes us avoid PREing calls that could trap across other
calls that might not return.  The PR88087 testcase has exactly
such case so I've refactored the testcase to contain a valid PRE.
I've also adjusted PRE to not consider pure calls possibly
not returning in line with what we do elsewhere.

Note we don't have a good idea whether a function always returns
normally or whether its body is known to never trap.  That's
something IPA could compute.

2021-09-01  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/93491
	* tree-ssa-pre.c (compute_avail): Set BB_MAY_NOTRETURN
	after processing the stmt itself.  Do not consider
	pure functions possibly not returning.  Properly avoid
	adding possibly trapping calls to EXP_GEN when there's
	a preceeding possibly not returning call.
	* tree-ssa-sccvn.c (vn_reference_may_trap): Conservatively
	not handle calls.

	* gcc.dg/torture/pr93491.c: New testcase.
	* gcc.dg/tree-ssa/pr88087.c: Change to valid PRE opportunity.